The present invention is a method and apparatus which enables the receiver of an airborne bistatic radar to estimate the state of an illuminator aircraft (position and velocity), by measuring and processing disjoint clutter cell backscattered Radio Frequency (RF) energy received at the receiver aircraft in an adaptive Kalman filter algorithm.
In a monostatic radar configuration, the transmitter and receiver are located at the same radar site. In this arrangement, the radar determines the slant range to the target directly from the time delay or phase delay of the received signal with respect to the transmitted signal. When the bistatic radar configuration is utilized, the radar transmitter and receiver are located at different radar sites.
The transmitter emits electromagnetic radiation signals having a time reference base, such as pulses (radar signals), and the receiver detects reflected radiation from targets illuminated by the transmitted radar signal. The range of the target may be determined by the time it takes a pulse of the electromagnetic radiation to travel from the transmitter to the target and then by reflection from the target back to the receiver. The transmitted pulses are focused in a narrow beam, and the bearing of the target is determined by the bearing of the transmitter's antenna at the time the reflected pulse is received. When used in military aircraft, monostatic radar has the disadvantage that the transmitter can be detected at long range (hundreds of miles) by the electromagnetic pulses it emits. , This allows the enemy to detect the presence of an aircraft and also to determine its bearing. To get around this disadvantage, bistatic passive radar was developed. Bistatic passive radar does not have a transmitter but rather has a receiver system that utilizes the radiation emitted by any monostatic radar system in its reception area. The transmitter of a monostatic radar system which is being used by a bistatic passive radar system is known as the host transmitter.
The airborne bistatic radar systems entail the use of two aircraft: the first aircraft possesses the radar transmitter and is referred to as an "illuminator"; the second aircraft possesses a passive bistatic radar receiver. The illuminator airpath is at comparatively moderate-to-high altitudes, and provides illumination of an enemy ground area of interest, the penetrator aircraft flies at comparatively low altitudes in order to avoid detection by the enemy.
The low altitudes planned for penetrator aircraft will often result in the penetrator not being within line-of-sight of the illuminator aircraft, which results in the need for some method of determining the illuminator's state (position and velocity). This task is alleviated, to some degree by the prior art techniques disclosed in the following U.S. Patents:
U.S. Pat. No. 3,487,462 issued to Holberg; PA0 U.S. Pat. No. 3,812,493 issued to Afendykiw et al; PA0 U.S. Pat. No. 4,246,580 issued to Caputi, Jr.; PA0 U.S. Pat. No. 4,325,065 issued to Caputi, Jr.; PA0 U.S. Pat. No. 4,370,656 issued to Frazier et al; and PA0 U.S. Pat. No. 4,456,862 issued to Yueh.
All of the patents listed above, with the exception of the Yueh patent, disclose bistatic synthetic aperture radar systems, and are incorporated herein by reference. Both of the Caputi patents, as well as that of Frazier et al, disclose airborne bistatic radar systems entailing a first aircraft possessing a long range radar transmitter, and a second aircraft with a receiver. The distance between the two aircraft is determined when the second aircraft receives direct path signals, which are received directly from the transmitter on the first aircraft. However, in order to receive direct path signals from the first aircraft, the second aircraft must fly with a sufficient altitude and position to maintain a line-of-sight contact with the first aircraft.
The problem the present invention seeks to solve is to enable airborne bistatic radar systems to function without the requirement of maintaining line-of-sight contact or the 1 necessity of receiving direct path signals. While the Holdberg invention is a bistatic radar system which does not require a reference direct path transmission, it is a land-based system in which the relative positions between the transmitter and receiver are known. Holdberg serves to clarify the problem the present invention is intended to solve: in order to make sense of received bistatic radar signals, the receiver needs to know the state of the transmitter.
The Yueh patent discloses a missile navigation system with 1 a Kalman estimator, in which the target is illuminated by the mother ship or launching platform. However, this still doesn't solve the present problem since the missile "knows", at least initially, the location of its point of launch.
The bistatic radar receiver of Afendykiw et al also relies on the traditional solution of receiving a direct path reference signal from the transmitting radar.
In view of the foregoing discussion, it is apparent that there currently exists the need for an airborne bistatic radar system in which the receiver aircraft is able to determine the illuminator aircraft's state without maintaining direct path reference signal contact with the illuminator aircraft. The present invention is directed towards satisfying that need.